# Two-equation turbulent models: low re airfoils

 Register Blogs Members List Search Today's Posts Mark Forums Read

 March 7, 2011, 06:03 Two-equation turbulent models: low re airfoils #1 Senior Member     Join Date: Jan 2011 Posts: 248 Blog Entries: 5 Rep Power: 10 Sponsored Links Hello I am trying to analyse flows past different 2D airfoils at low reynolds numbers (order of 10^4-10^5) and having problems. I am using fluent, but I guess this is not the fluent problem, but rather a general one. At this reynolds number significant part of the flow over an airfoil is laminar and separation bubbles with transition are formed. First I run one-equation SA model, which obviously produces turbulent flow over whole airfoil, as there were trips at the leading edge, and generally the drag is overestimated. Here everything is going smooth, but solution is nof of inerest. At these reynolds numbers you can get a fine mesh without problems, as boundary layer is thik and laminar sublayer can be resolved without wall functions. In my case y+ is order of 1. Then, to take separation and transition into accound I am trying two equation k-omega and k-omega sst transitional models. Here I am going into the trouble as solution never converges: choise of different discretization schemes, solvers, relaxation parameters, furter refinement of the grid does not help. Is it a typical problem with two-equation models at this reynolds number? I was trying to search on the web, but information on application of RANS for these reynolds numbers is quite poor. If there is somebody who had experience in this kind of simulation: what solver discretization scheme and relaxation parameters to use? which initial conditions to set? which boundary condition for k and omega to set? (I set boundary condition for turbulent quantities based on my x-foil experience: clculated from turbulence intensity of 0.07% and lenght 0.01m at inlet and outlet) Will be grateful for your help Truffaldino

 March 7, 2011, 12:48 #2 Senior Member     Join Date: Jan 2011 Posts: 248 Blog Entries: 5 Rep Power: 10 I have managed to make to convergence k-omega standard transitional model by iterating standard k-omega until convergence and then running k-omega standard transitional, but results are dissapointing: the drag coefficient 3.5 times higher than experimental and lift coefficient 10% lower. Still, k-omega SST diveres whatever I am trying to do Any help? Truffaldino

 March 7, 2011, 19:02 #3 Senior Member     Vieri Abolaffio Join Date: Jul 2010 Location: Always on the move. Posts: 308 Rep Power: 10 try to first obtain convergence with k-e or k-omega and just then switch to sst. sometimes it might work. using sst from the beginning of the simulation is really hard in my opinion. let me know if it help

March 8, 2011, 04:08
#5
Senior Member

Join Date: Jan 2011
Posts: 248
Blog Entries: 5
Rep Power: 10
Quote:
 Originally Posted by sail try to first obtain convergence with k-e or k-omega and just then switch to sst. sometimes it might work. using sst from the beginning of the simulation is really hard in my opinion. let me know if it help
Thank you for suggestion, swithching

k-omgega ---> k-omega sst ----> k-omega sst transitional

does not help: k-omega goes fine, then k-omega sst still have acceptable residuals, but when finally switching to transitional residuals are oscillating wildly. Perhaps I am using a wrong discretization scheme. Which discretization schemes do you suggest?

Last edited by truffaldino; March 8, 2011 at 06:16.

March 8, 2011, 04:21
#6
Senior Member

Join Date: Jan 2011
Posts: 248
Blog Entries: 5
Rep Power: 10
Quote:
 Originally Posted by Martin Hegedus Why do you expect your solution to converge, i.e. be steady? I assume that is what you mean, i.e. converge to steady state.
No, I mean numeric convergence, i.e. reasonable decrease in resudual. I have tried both stationar and non-stationar solvers: residuals oscillate wildly in kw-sst transitional model. Perhaps indeed the eddy viscosity is too low in k-w sst transitional to de-chaotise the solution. Perhaps I have to set higher turbulence at inlet to see if it will make solution to converge.

Last edited by truffaldino; March 8, 2011 at 06:17.

March 8, 2011, 14:13
#7
Senior Member

Vieri Abolaffio
Join Date: Jul 2010
Location: Always on the move.
Posts: 308
Rep Power: 10
Quote:
 Originally Posted by truffaldino Thank you for suggestion, swithching k-omgega ---> k-omega sst ----> k-omega sst transitional does not help: k-omega goes fine, then k-omega sst still have acceptable residuals, but when finally switching to transitional residuals are oscillating wildly. Perhaps I am using a wrong discretization scheme. Which discretization schemes do you suggest?

simple, second order everything, green-gauss node based, double precision, might require some fiddling with the under-relaxation factors.

can you post a picture of your mesh? with a close up of the trailing edge if possible. just to check that you don't have highly skewed elements due to a sharp trailing edge angle.

also, you might want to check the residuals location in your domain to see if it might influence the solution or not (not sure it might be done in fluent)

might i ask how much the residuals are oscillating? everything or just some values? how many iterations do you use?

 March 8, 2011, 14:30 #8 Senior Member   Martin Hegedus Join Date: Feb 2011 Posts: 480 Rep Power: 12 I gather what you are saying is that, even for an unsteady problem, your residuals have not converged enough. Since you mentioned separation bubble one of my thoughts is that your k-omega sst transitional model does not have enough eddy viscosity to stabilize the bubble, and the bubble is busting, reforming, bursting, ... Some additional questions: 1) What is the thickness of your airfoil? 2) What angle of attack are you running? 3) Have you tried running laminar and comparing the solution behavior to your k-omega sst transitional model? 4) Did your k-omega and k-omega sst models converge to a steady or unsteady solution? Edit: Can you also show us a plot of the eddy viscosity around your airfoil for either the k-omega or k-omega sst models. Thanks.

 March 8, 2011, 14:44 #9 Administrator     Jonas Larsson Join Date: Jan 2009 Location: Gothenburg, Sweden Posts: 569 Rep Power: 10 Trying to predict laminar separation, natural transition and turbulent reattachment using a two-equation turbulence model is a bit optimistic! From my experience two-equation turbulence models like k-eps, k-omega, SST k-omea, ... should only be used when you have fully turbulent boundary layers. The same goes for the SA model. If you want to solve transition you need some form of special transition model or correlation. Some researchers claim to be able to predict by-pass transition using only two-equation models. Natural transition can never be predicted with a normal two-equation turbulence model! Note the difference between by-pass transtion, caused by diffusion of turbulent energy into the boundary layer from a turbulent free-stream, and natural transition, caused by instabilities in a laminar boundary/shear layer. However, I do not believe that by-pass transition can be reliably predicted using just a two-equation turbulence model. Sometimes you can predict a transitional behaviour, but it does not occur at the correct position and often does not show the correct physical characteristics. With Fluent most of the two-equation models can not even predict a transitional behaviour. You will get turbulent boundary layers right from the leading edge. The only model implemented in Fluent which I have been able to get any transitional behaviour with is the low-Re Launder-Sharma k-epsilon model. ajay17 likes this.

March 8, 2011, 16:57
#10
Senior Member

Join Date: Jan 2011
Posts: 248
Blog Entries: 5
Rep Power: 10
Quote:
 Originally Posted by jola With Fluent most of the two-equation models can not even predict a transitional behaviour. You will get turbulent boundary layers right from the leading edge. The only model implemented in Fluent which I have been able to get any transitional behaviour with is the low-Re Launder-Sharma k-epsilon model.
Yes, I have understood this by looking at result of computation. As far as I undersand it is necessary to introduce user defined intermediency factor or other "artificial" things like that that are uncontrolled and highly unreliable.

Before, I was using x-foil for this kind of analysys and it a way much better than using turbulence models on mesh for airfoil analysys in this range of low reynolds numbers:

My problem is that I want to do an analysys of stepped airfoils, for which x-foil is not suitable, so I decided to try CFD on mesh. To validate the method and get some training I started with conventional airfoils, and it turns out that 2eqn turbulence modelling is not suitable even for them, not to mention stepped airfoils I was going to analyze in prospective!

Perhaps one should use LES in this situation?

 March 8, 2011, 17:13 #11 Administrator     Jonas Larsson Join Date: Jan 2009 Location: Gothenburg, Sweden Posts: 569 Rep Power: 10 Using CFD and normal turbulence models to predict laminar separation/natural transition/turbulent reattachment is VERY difficult and not something that can be done reliably in even very controlled research cases. LES can be used and is used in research. But to do this reliably you need to use some form of transition prediction method or correlation that has been validated for geometries and conditions similar to the one you want to predict. I do not know x-foil, but I assume that it includes some form of correlation to predict this which has been validated on similar cases. I would recommend you to start looking for simple correlations to predict these phenomena and use these correlations to control a user-defined intermittency factor or similar in your CFD code, as you describe. Hence, my recommendation is ad-hoc experimentally validated correlations instead of trying more advanced general CFD methods (LES etc.)

March 8, 2011, 17:15
#12
Senior Member

Join Date: Jan 2011
Posts: 248
Blog Entries: 5
Rep Power: 10
Quote:
 Originally Posted by Martin Hegedus Some additional questions: 1) What is the thickness of your airfoil? 2) What angle of attack are you running? 3) Have you tried running laminar and comparing the solution behavior to your k-omega sst transitional model? 4) Did your k-omega and k-omega sst models converge to a steady or unsteady solution?
This is sd7037 airfoil (9% thikness) at Re=70000. Sure, there is a separation bubble as you can clearly see from X-foil run shown below: that is why laminar solution never converges, as you have noticed. I will try to upload fluent plots and mesh a bit later.

k-omega converges very well for steady simulation (I set residuals 10^(-6)), k-omega sst oscillates at higher residuals and never reaches 10^(-6).
Attached Images
 sd7037_xfoil.jpg (32.9 KB, 235 views)

March 8, 2011, 18:12
#13
Senior Member

Join Date: Jan 2011
Posts: 248
Blog Entries: 5
Rep Power: 10
Quote:
 Originally Posted by Martin Hegedus one of my thoughts is that your k-omega sst transitional model does not have enough eddy viscosity to stabilize the bubble
Thanks Martin,

Seems you are right, see plots for steady simulation form kw (2000 iterations) to kw sst staedy with oscillating residuals (they are also shown). The eddy viscosity is too small. But I am wandering, why then kw-standard converges so good, but overestimates the drag order of magnitude? What turbulence intensity do you sugges at the inlet?

In my case I set intensity 0.07% and turbulence lenght is an airfoil chord.
Attached Images
 Turbulent viscosity kw.jpg (20.7 KB, 168 views) turbulent_viscosity_kw_sst.jpg (21.1 KB, 152 views) residuals_kw_to_kw_sst.jpg (27.0 KB, 176 views)

March 8, 2011, 18:27
#14
Senior Member

Join Date: Jan 2011
Posts: 248
Blog Entries: 5
Rep Power: 10
Quote:
 Originally Posted by sail can you post a picture of your mesh? with a close up of the trailing edge if possible. just to check that you don't have highly skewed elements due to a sharp trailing edge angle.
Thank you a lot! I post a picture of the mes and closeups of LE and TE. Plot of residuals is shown in the previous post.

Truffaldino
Attached Images
 Mesh.jpg (98.8 KB, 227 views) airfoil_mesh.jpg (98.4 KB, 225 views) airfoil_Leading_Edge.jpg (50.4 KB, 187 views) airfoil_Trailing_edge.jpg (49.5 KB, 186 views) kyplus kw.jpg (24.3 KB, 191 views)

March 8, 2011, 18:39
#15
Senior Member

Join Date: Jan 2011
Posts: 248
Blog Entries: 5
Rep Power: 10
Quote:
 Originally Posted by jola I do not know x-foil, but I assume that it includes some form of correlation to predict this which has been validated on similar cases. I would recommend you to start looking for simple correlations to predict these phenomena and use these correlations to control a user-defined intermittency factor or similar in your CFD code, as you describe. Hence, my recommendation is ad-hoc experimentally validated correlations instead of trying more advanced general CFD methods (LES etc.)
The problem here that there is no experimental results for stepped vortex-trapping airfoils, analysys of which is a final goal of thisn work. I wanted to compare performance of this type of airfoils with best conventional ones (and now trying to analyse only conventional ones using CFD code). For conventional airfoils one can establish correlations etc, but these data are of no use in different geometry of vortex-trapping airfoils.

As for x-foil: it uses viscous-nviscid interaction for boundary layer through integral BL methods, transition is predicted by e^n method. Program seems to use some othrer correlations (I am not copletely sure).

Last edited by truffaldino; March 9, 2011 at 01:24.

March 9, 2011, 02:35
#17
Senior Member

Join Date: Jan 2011
Posts: 248
Blog Entries: 5
Rep Power: 10
Quote:
 Originally Posted by Martin Hegedus Instead, do a 2d contour plot of the eddy viscosity in the region near the airfoil.
Yes, I forgot that in k-w there is zero boundary condition for eddy viscousity.

These are contours of turbulent viscosity in the fluid. For kw steady (that converges) and kw-sst steady that oscilates

People advice me not to switch form model to model, but run transient with a fixed model. If it is a good way to fix things?
Attached Images
 kw_Turbulent viscosity_contours_mesh.jpg (25.1 KB, 139 views) kw_Turbulent viscosity_leadin edge.jpg (28.9 KB, 106 views) kw_Turbulent viscosity_trailing_edge.jpg (35.5 KB, 112 views) kw_sst_Turbulent-viscosity.jpg (31.1 KB, 139 views) kw_sst_Leading_Edge.jpg (31.7 KB, 110 views)

March 9, 2011, 02:45
#18
Senior Member

Join Date: Jan 2011
Posts: 248
Blog Entries: 5
Rep Power: 10
here is the close up for kw-sst for trailing edge. Also residuals for

Time step: (1% of airfoil chord flow travel per time step, max 10 iterations per step).
Attached Images

March 9, 2011, 04:04
#19
Senior Member

Join Date: Jan 2011
Posts: 248
Blog Entries: 5
Rep Power: 10
And here is eddy viscosity from S-A models, where everything is going smooth
Attached Images
 SA_turbulent_viscosity.jpg (30.5 KB, 65 views) SA_turbulent_viscosity_closeup.jpg (39.6 KB, 92 views)

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Jade M Main CFD Forum 27 August 11, 2017 15:41 doug Main CFD Forum 6 August 4, 2012 14:39 makaveli_lcf OpenFOAM Running, Solving & CFD 0 June 8, 2009 09:34 Flo.duck Main CFD Forum 0 May 6, 2009 03:37 Andrea CFX 2 October 11, 2004 05:12